Pigment production in vitro was successfully stimulated in S91 melanoma cells by treatment with MSH and to a lesser extent with ACTH. Estradiol and testosterone propionate did not significantly effect pigment production or growth. Growth was inhibited with MSH when the cell density was high but plating efficiency was increased at low cell densities. Tumorigenicity was similar in the B versus BV cell lines except at very low cell inocula. NCTC cells, a different tissue culture line of S91 melanoma did not produce tumors at low inocula. Chromosome analyses indicate a much wider range of chromosome numbers for the long term in vitro B cells as compared to BV cells. The B line is hypotetraploid, lacking one pair of chromosomes, while the BV cells have an additional pair. Human melanomas in vitro are now being studied following treatment with ACTH and MSH in an attempt to establish a human cell model to determine pigment-production mechanisms and their possible relationship to growth. Experiments are planned to combine hormone (ACTH or MSH) plus each of the following molecules: a) actinomycin D, b) puromycin, c) alpha- amanitin, d) cordycepin, e) cytochalasin B, and f) vinblastin and to note changes in pigment-production. Such data may help to explain the molecular mechanisms involved in visible melanin formation and also growth.